Adsorption and Retention of U-238 and Th-232 from Groundwater Using BIOS
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$GVRUSWLRQDQG5HWHQWLRQRI8DQG7KIURP*URXQGZDWHU8VLQJ%,26 Craig R. Anderson and Karsten Pedersen Deep Biosphere Laboratory, Department of Cellular and Molecular Microbiology, University of Gothenburg, Box 462, SE-405 30, Gothenburg, Sweden $%675$&7 Biofilms were grown LQVLWX 296 metres below sea level in the Äspö Hard Rock Laboratory. The prominent organism in these biofilms was *DOOLRQHOODIHUUXJLQHD, which is an iron oxidising chemolithotrophic microorganism that grows in low oxygen conditions. This organism grows an organic stalk structure capable of binding and concentrating trace metals. This stalk structure also allows amorphous ferric iron oxyhydroxides, or BIOS, (bacteriogenic iron oxides) to precipitate. The pH of the groundwater within the system was between 7.4 and 7.6, with Eh potential between 150 and 190 mV and oxygen saturation between 3 and 15%. Biofilms developed within two weeks and were sampled every two weeks for three months. Cell number and stalk length was recorded for each sample. The concentration of Cr, Ni, Cu, Zn, Mo, REE (rare earth elements), U-238 and Th-232 was measured by ICP-MS. Early results suggested that *DOOLRQHOODbiofilmsand associated BIOS could potentially concentrate trace metals up to 1000 fold higher than levels within the host rock and over 1 000 000 times the levels in the groundwater over a period of years. These new experiments indicate that *DOOLRQHOOD biofilms and BIOS can rapidly attenuate metals to levels over 1000 fold higher than the levels in the groundwater. This process can occur anywhere where reduced groundwater enters the waste repository tunnel, open cavities or where groundwater extrudes at the surface. Because of this, there is huge potential to use biofilms and BIOS for retention of radionuclides and pollution control. ,1752'8&7,21 Mechanisms of radionuclide dispersion are an important consideration in the design and operation of high-level nuclear waste repositories [1]. Investigations of colloids, including bacteria, are needed to gauge the positive and negative effects. Colloids are important because they have a large reactive surface area and are ubiquitous[2]. Both organic surfaces and iron oxides have been identified as being important in regard to their capacity and efficiency to absorb metals. The contribution of bacteria has been underestimated as they can significantly enhance Kd (distribution coefficients) values[3]. *DOOLRQHOODIHUUXJLQHD is an iron oxidising chemolithotrophic microorganism that produces biofilms in micro-aerophilic to oxidised conditions. The organism produces a stalk structure from the concave side of the cell, depending on cell number, pH and redox conditions[4]. These extracellular stalk structures are the predominant biomass within the biofilm[5]. After the cells oxidize ferrous iron, BIOS (bacteriogenic iron oxides) precipitate on the stalk material. Over time the stalks and associated BIOS can attenuate large concentrations of metals and radionuclides from surrounding groundwater[6]. To investigate
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